The present invention relates to a method for the manufacture of diaphragms for electrochemical cells by impregnating asbestos paper with an organic plastic, and to asbestos diaphragms prepared by such a method.
In addition to sufficient porosity and high chemical resistance to the media used, diaphragms for electrochemical cells must, in particular, also exhibit high mechanical strength. Accordingly, it is known to use in fuel cells asbestos diaphragms which contain, for example, high-purity asbestos fibers and plastics, in which the plastic is a binder of methacrylic acid ester, polysulfones, chlorosulfonated polyethylene or chloroprene in amounts of 0.5 to 6% by weight, in particular, 1.5 to 3% by weight, referred to the weight of the asbestos fibers (British Patent No. 1,213,472). In the preparation of the asbestos diaphragms, the plastic is added to the asbestos fibers in the form of an aqueous dispersion.
U.S. Pat. No. 3,583,891 describes, for use in electrochemical cells, particularly fuel cells, a gas-tight membrane of high mechanical strength of asbestos or carbon material in fiber form and a plastic binder in which, in the fabrication of the membrane from the fiber material, a butadiene-styrene-acrylonitrile copolymerizate is added to the former (copolymerizate content of 6 to 15% by weight, referred to the fiber material) and wherein the nitrile groups of the copolymerizate are saponified to carboxyl groups after the membrane has been formed.
The methods mentioned are not well suited for the efficient manufacture of asbestos diaphragms for electrochemical cells, however, since they are relatively expensive. In general, the manufacture of diaphragms with an asbestos fiber base also is known, where asbestos paper is impregnated with a plastic material and is subsequently heated to remove the solvent or dispersing agent for the plastic material.